Mercury investment casting alloy composition



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C. A. HOAG MERCURY INVESTMENT CASTING ALLOY COMPOSITION Filed Dec. 18.1961 for' Power SUPP/y A40/a emova/ INVENTOR.

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United States Patent 3,212,885 MERCURY INVESTMENT CASTING ALLOYCGMPOSITION Chauncey A. Hoag, Claremont, Calif., assignor to MertronicsCorporation, Santa Monica, Calif., a corporation of Delaware Filed Dec.18, 1961, Ser. No. 160,143 3 Claims. (Cl. 75-169) This invention relatesgenerally to the art of forming and using investment casting patterns,and more particularly concerns novel alloy compositions and methods forforming and using patterns made therefrom.

U.S. Patent 2,857,641 describes the desirability of using intermetalliccompounds of mercury and thallium for forming investment castingpatterns. An alloy was indicated that is close to the thallium-mercuryeutectc composition, Tl2Hg5, which contains 28.95 percent by weight ofthallium and which starts to freeze at about 56 degrees F. and which issolid within one degree range. No other metallic composition thatfreezes :so slightly below -ordinary room temperature was thenavailable. This alloy was attractive for the indicated use because ofthe potentially low refrigeration requirement. However, thalliumoxidizes readily and the oxide is Very soluble in water, forming analkaline hydrate solution that is a good conductor. Thallium iselectro-positive to most common metals and decidedly so to mercury. Thepatent specilies the use of a protective liquid covering to suppressoxidation of the thallium and suggests ethylene glycol as the preferredliquid. But, under use conditions atmospheric moisture is condensed onmetal parts and is absorbed by the ethylene glycol bringing free oxygeninto contact with the alloy and starting the above chain of events.

As a result, the protective uid soon had to be replaced. Depletion ofthallium from the alloy made it necessary to replace the expendedthallium frequently and chemical analyses and computations had to bemade to determine the amount of thallium to add each time. Sinceethylene glycol costs about $4.50 per gallon and thallium costs about$12.50 per pound, the maintenance costs were very high relative to anypotential savings in yrefrigeration costs. Further, as the thalliumbecame depleted the frozen patterns made from it became increasinglydiicult to book securely.

The present invention seeks to overcome the above mentioned problemsthrough an approach which places primary reliance upon a factor otherthan the use of a protective liquid. As will appear, the inventionbroadly concerns the addition to the alloy of other metallic substancesuch as zinc which is electro-positive to thallium. As will be broughtout, the zinc is preferably present in dissolved state in the alloy andin amount suflicient to inhibit oxidation of the thallium. Since thezinc oxidizes in preference to thallium, the dissolved zinc content ofthe alloy will tend to be depleted, and the invention contemplateskeeping solid state zinc bodies in the liquid alloy reservoir from whichpattern forming alloy is withdrawn for replenishment of the dissolvedzinc metal whi-ch becomes oxidized.

Still another aspect of the invention has to do with the method by whichthe preferred alloy composition may be kept available for use in patternforming and without deterioration, the method facilitating delivery ofpure alloy of the correct composition at any desired time and alsocleaning of the fluid alloy.

These and other objects of the invention as well as the details of anillustrative embodiment will be more fully understood from the followingdetailed description of the drawing, in which:

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FIG. 1 is a flow diagram illustrating the use of the alloy; and

FIG. 2 shows the manner in which the alloy may be kept for maintenanceof correct composition, and for delivery of pure alloy at any desiredtime.

Referring rst to FIG. 1, typically the alloy withdrawn or delivered at12 is used 4to invest dies or master molds which have previously beenlled with acetone for absorbing moisture-free contact with the dieinterior surfaces, this step being indicated at 14. The alloy containedin the dies is then frozen as indicated at 15, after which the die isstripped from the frozen alloy pattern sections. The latter are thentreated with a suitable ux for purposes of facilitating ready booking ofthe pattern sections as indicated at 16. The patterns are then rinsedfree of excess flux solution, as by placing them in an ice water bath,serving for temporary storage to allow efective diffusion of the excessflux. Just prior to use of the patterns for molding purposes, they aredried as by successive rinsing in acetone.

Finally, the booked patterns are used to formmolds at 17, and typicallyceramic spray in a Freon carrier is used to build up successive layersof the mold on the frozen alloy pattern, the spray booth beingdehumidied -to prevent the condensation of atmospheric moisture on thesuccessive layers .of ceramic mortar. Even if the films of moisture donot freeze, they impair the bonding between layers and producenon-uniform density and weak molds prone to separation during curing.(The main curing and conditioning of the molds takes place in thefoundry where the molds are gradually heated to high temperature that isrelated to the pouring temperature of the metal to fbe used forinvestment casting.) The patterns and molds are then transferred at 18to a melting chamber at 10 in FIG. 1 (22 in FIG. 2) wherein the frozenalloy is melted out of the molds. The molds are then withdrawn asindicated at 11 for subsequent use in casting, and the melted alloy isremoved at 12 for reuse in process. The numeral 13 indicatesreplenishment of liquid alloy and zinc in such amounts as are foundnecessary.

In accordance with one phase of the invention, the thallium content ofthe alloy is protected against oxidation by the addition to the alloy of`a metallic substance that is electro-positive to thallium. In thisregard, the thallium present in the alloy is sufficient to elevate thefreezing temperature of the alloy well above the freezing temperature ofmercury alone, and typically the thallium content will approximate thestoichiometric content of the eutectic intermetallic alloy Tl2Hg5containing 28.95 percent by weight of thallium. This alloy freezessolidly rat about 56 degrees F., which is within Ithe range of ordinaryrefrigeration making the alloy attractive for commercial use. i

The metallic substance preferred for addition to the mercury thalliumalloy is zinc, although other substances electro-positive to thalliumare workable if not as prac@ tical as zinc, other such substancesincluding magnesium. Sodium and potassium are believed impractical fromthe standpoint of their rapid reaction with moisture leading to rapiddepletion of their content and effectiveness in the alloy to preventoxidation of the thallium.

The zinc dissolved in the alloy may be about 3 percent by weight whenprepared at near the boiling point of water, but at ordinary roomtemperatures it is typically close to 0.6 percent by weight in the duidalloy, this content being maintained in the process by the addition 0fsolid state Zinc particles as shown at 19 in FIG. 2 to the liquid alloypool 20 formed in a sink 21 at the bottom of the melting chamber 22.With zinc in solution in the alloy, thallium is protected againstoxidation and dissolution, and no oxidation suppressing organic liquidsuch as ethylene glycol is needed. Zinc is oxidized to some extent, butzinc oxide is almost completely insoluble in water.

If one should ever need to increase the potential for rapid replacementof zinc in the alloy at ordinary temperatures, he may, at or near theend of a work shift, heat the pool of alloy in contact with excess zincmetal chunks for a period of about two or three hours at a temperatureonly a little below the boiling point of Water. He may then allow thealloy to cool to room ternperature, before the next work shift, at whichtime he will have a crop of tiny zinc rich alloy crystals in the upperlayer of the pool, and these collectively have a vastly greater surfacearea than the solid pieces of zinc from which they were derived. Byreason of the increased area of contact between fluid alloy and zincplus zinc rich crystals, the potential for rapid replacement of depletedzinc is considered substantially increased.

The numeral 23 indicates a mold from which a frozen alloy pattern 24 isbeing melted through the application of heat to the chamber as indicatedby the electrical heating system 25. A screen is shown at 26 tointercept pieces of ceramic which might otherwise fall from the mold 23into the pool 20. At the top of the pool there is formed a layer 27 offloating impurities including zinc hydrate or hydroxide, the layer 27also including fine ceramic debris and water covering the pool toprotect it from oxidation by the atmosphere within the charnber 22. Theoxygen content of the water layer reacts with the Zinc dissolved in thealloy to form the zinc hydrate, and accordingly the thallium content ofthe alloy is not oxidized but remains at the preferred weight percent.

As the level of the pool tends to rise in response to melting out ofpattern alloy from the molds 23, the liquid alloy in a trap conduit 28will be displaced into a reservoir 29, the alloy content 30 of which mayvary. Conduit 28 is shown as communicating with opening 31 in the bottomof the sink 21, the conduit having a gooseneck portion 32 in which thelevel 33 of the alloy remains the same as the surface level of the alloyin the pool 20. Accordingly, the liquid alloy 20 never cornpletelydrains from the sink 21, and the layer 27 is never carried over into thereservoir 29. Alloy in the gooseneck riser 34 will be displaced overinto the receptacle 29 with the addition of alloy to the pool 20. Aprotective liquid such as water 35 is shown covering the surface 33 ofthe alloy in the gooseneck and also as covering the alloy 30 in thereservoir 29. The alloy liquid 30 in the reservoir 29, after mixing, maybe withdrawn from the cone shaped bottom of the reservoir as by thevalve apparatus shown at 36 whenever desired to ll the master dies. Oneform of mixing apparatus includes a rotor 37 rotatablek in the alloypool 30 as by a suitable drive 38 and motor 39. The rotor shouldpreferably be close to the bottom outlet 43 so that even small batchesof alloy will be mixed. Also, the rotor should be driven rather slowlyso as not to cause separation of the alloy into droplets, and the rotordesign should be such as to give a slight upward thrust or circulationto the alloy, assuring mixing of the heavy'alloy. A perforated plate isshown at 40, typically near the outlet 31 of the sink 21 in order tostrain any particle impurities from the alloy flowing downwardly fromthe pool 29 and into the trap conduit 28. Typically, the plateperforations may comprise holes of about 1/32 inch diameter.

The following table shows the results of freezing tests conducted upon anumber of batches of alloy with varying weight percentages of thallium.In all of these tests the remaining weight percentage content of thealloy other than thallium comprises mercury containing 1 percent zinc,so that the zinc content varies from .73 percent for batch Y319 to .50percent for batch G322.

Freezing Batch Thallium, Initial RP., Final RP Range,

Percent Deg. F. Deg. Approx.,

Deg. Range 26. 7 4s. s 26, s 12 27. 9 50. 6 42 9 28. 3 50. 9 45 0 2s. 751. 4 47 4 29. 1 5l. 2 48 3 30. 2 51. 7 48 4 3l. 6 5l. 2 47 4 32. 0 50.9 37. 6 1S 33. 0 49. 7 30. 2 19 34. 0 48. 3 35. 3 13 34. 6 44. 6 34. 410 38.8 39. 2 30 9+ 40. 3 40. 5 30. 4 10 It was found that the thalliumalloys with relatively wide freezing temperature range, that is thealloys containing 32.0 percent thallium and greater, have rough spots ortears of extruded alloys on the surface of the patterns, these roughspots being the last to freeze. Furthermore, if such alloys are used formaking molds, the castings resulting from use of the molds have a roughsurface, and accordingly it is determined that the alloy compositionshould be such that the freezing temperature range is considerably lessthan 13 degrees F., or alternatively that the thallium content should beno more than 31.8 weight percent.

Also, it was found that pattern sections containing 27.9 and less Weightpercent thallium not only have a freezing temperature range of 9 degreesF. and greater, but also exhibit inferior booking properties. Forexample, whereas the alloys having freezing ranges of no more than 6degrees F. may be booked rmly without external pressure applied to thetest pieces, it is necessary to apply external pressure to the testpieces in order to book them when such pieces contain thallium outsidethe pre ferred freezing temperature range of no greater than 6 degreesF.

Referring. again to the dissolution of high purity electroyltic zincmetal in the mercury thallium alloy it was found that excess zinc metalmay be kept floating in the fluid alloy at ordinary temperatures withoutputting an excess of zinc into the alloy, i.e. the zinc content remainsbelow 1.0 percent by Weight. At the start, zinc is incorporated in thethallium-mercury alloy up to the equilibrium content at roomtemperature, ordinarily 0.6 to 0.7 percent by weight of zinc. This alloystarts to freeze at about 5l degrees F. and becomes solid at about 47degrees F. Refrigeration requirements are therefore similar to those forthe straight thallium-mercury alloy. Under operating conditions the zinccontent of the alloy maybe somewhat depleted, in which case the initialfreezing point would be higher than 5'1 degrees. However, so long as theinitial freezing point of the alloy remains below 56 degrees F., aneifective zinc content in the alloy is assured.

More rapid wetting of the zinc pieces by the fluid alloy is attained byapplying acetate flux solution to the pieces, facilitating thedissolution of the zinc in the alloy. Also, the application of lgentleheat and stirring lpromote dissolution as indicated by the heater 41 andpower driven mixer 42. Another advantageous function of the zinc is toneutralize any acid materials, such as residues from Freon used inceramic sprays falling into the pool 20 from the mold 23 during themelting process. A simple test to determine whether the zinc content ofthe liquidalloy has been exhausted consists in mixing a portion of thealloy `with water and adding phenolphthalein indicator. If zinc has beenexhausted, thallium will go into solution, the water will be alkalineand the indicator will show red coloration immediately.

It should be pointed out that segregation of the components of theliquid alloy takes place due to differences in density between mercury,thallium and zinc. To assure ne structure and smooth surfaces on thefrozen .patterns it is essential to thoroughly mix the iuid alloyforming a shell mold, said alloy consisting of before it is dispensed.liquid mercury,

I claim: thallium mixed with the liquid mercury, said thallium 1. A lowmelting investment alloy for use in forming being present in a suicientquantity to raise the a shell mold, said alloy consisting of 5 meltingpoint of the yalloy solution into a range bemercury in arange of between68 and 73% by weight, tween 40 F. and 56 F., and thallium in the rangebetween 28 and 32% by weight, a metal that is electro-positive tothallium mixed in and said solution, said metal being in the range of0.5 zinc between 0.5 and 3% by weight. up to 3% from a class consistingof zinc, magnesium, 2. A low melting investment alloy for use in forming10 a portion of said met-al being dissolved in said alloy a shell mold,said alloy consisting of solution and the `remaining portion being insolid mercury and thallium, form. said thallium being present in a rangethat will cause initial freezing to begin at less than 56 and theReferences Cited by the Examiner final freezing to be completed above 40F. and in 15 UNITED STATES PATENTS a range that Will cause thedifference between the initial freezing and the nal freezing to be lessthan 2857641 10/'58 Kramer 75-169 X 6 F, and b t 0 5 nd 3l? b ht FOREIGNPATENTS zinc 1n a range e Ween a o y we1g the remainder of .said alloybeing all mercury. 1010743 6/57 Germany 3. A low melting investmentalloy solution for use in DAVID L. RECK, Primary Examiner.

1. A LOW MELTING INVESTMENT ALLOY FOR USE IN FORMING A SHELL MOLD, SAIDALLOY CONSISTING OF MERCURY IN A RANGE OF BETWEEN 68 AND 73% BY WEIGHT,THALLIUM IN THE RANGE BETWEEN 28 AND 32% BY WEIGHT, AND ZINC BETWEEN 0.5AND 3% BY WEIGHT.